The present invention relates to a power threshold optoelectronic switch, as well as to its control process.
The present invention is applicable to very broad fields, such as laser system instrumentation, particularly for synchronizing different optical and electrical signals for the control of Pockels cells, for cutting off or extracting laser pulses.
An optoelectronic switch is a device making it possible to establish, under the effect of an incident light beam, an electrical conduction between two normally insulated electrodes.
FIG. 1 diagrammatically and sectionally shows a known optoelectronic switch. This switch comprises a layer 3 constituted by a semiconductor photoconductor material having a thickness of e.g. approximately 100 .mu.m, placed between a first and optionally second substrates 2, 1. The first substrate 2 is constituted by ambient air and the second substrate 1, when it exists, is constituted by an insulating material, such as alumina.
This switch also comprises first and second normally insulated electrodes 5, 7 deposited on the upper surface of layer 3, as well as a reference electrode 9 deposited over the entire lower surface of substrate 1, when it exists, or if substrate 1 does not exist over the entire lower surface of layer 3, forming with electrodes 5 and 7 a transmission line. The first and second electrodes 5, 7 are aligned and spaced so as to form an interelectrode gap 11.
Upper surface as compared with the lower surface is understood to mean the surface closest to the incident light beam.
Electrode 5 is connected to means 10, such as a voltage generator, making it possible to apply a supply voltage to said electrode 5. Electrode 7 is connected to observation and/or utilization means 17 having an impedance matched to the impedance of the transmission line formed by electrodes 5, 7 and 9. This impedance is generally a resistance of 50.OMEGA.. Moreover, means 10, 17 and reference electrode 9 are connected to a reference ground.
The observation and/or utilization means 17 collect the electric signal received by electrode 7, when an electric conduction is established between electrodes 5 and 7. This electric signal corresponds to the output voltage of the switch.
To establish an electric conduction between the two electrodes 5, 7, an incident, pulse-type light beam 13 from a light source, such as a laser source, is supplied to the layer 3 in the interelectrode gap 11. The wavelength of beam 13 must be such that the energy of the photons of said beam is located in the absorption band of the material forming layer 3, or is adjacent to the edge of said band. For a silicon layer, it is e.g. possible to choose a light source, such as a neodymium laser with a wavelength of 1.06 .mu.m.
Under these conditions, that part of the light beam 13 which penetrates layer 3 is partly absorbed in the latter. During its passage in layer 3, the absorbed fraction of beam 13 produces electron--hole pairs in the material forming said layer 3. These electron--hole pairs are charge carriers making it possible to establish an electric conduction between the two electrodes 5, 7.
When there are sufficient such charge carriers, they make it possible to establish a short-circuit between the two electrodes 5, 7, so that there is an electrical continuity between said electrodes.
The elimination of the light beam 13 transmitted into the interelectrode gap 11 makes it possible to again electrically insulate the two electrodes 5, 7, after the charge carriers have disappeared by recombination.
The rising front of the electric signal collected by the observation and/or utilization means 17 is dependent on the shape of the light signal of the incident beam 13. Generally the shape of this light signal is not reproducible, which also applies to the shape of the electric signals supplied by electrode 7 during the various uses of the switch, so that an uncertainty exists regarding the switch tripping time during each use of said switch. The term tripping of the switch is understood to mean the time at which electrodes 5, 7 are short-circuited.
This lack of reproducibility of the electric signal, obtained from the pulse-type light beam, can in the case of the use of the switch for synchronizing observation equipment, lead to synchronization faults, which can be of the same order of magnitude or even greater than the characteristic times of the phenomena to be studied. Moreover, the synchronization problems become more critical as the light pulses become shorter, e.g. approximately 1 nanosecond or one picosecond.
Moreover, when the reference electrode 9 is deposited on the lower surface of layer 3, or in other words when the switch has no substrate 1, the formation of charge carriers throughout the thickness of layer 3 can lead to a short-circuit between electrodes 5, 7 and reference electrode 9. The significant thickness of layer 3 also does not make it possible to retain a constant impedance of the transmission line formed by electrodes 5, 7 and 9 and this causes disturbances to the electric signal.